Numerical Prediction of the 10–11 June 1985 Squall Line with the Canadian Regional Finite-Element Model

Abstract

In an effort to improve operational forecasts of mesoscale convective systems (MCSs), a mesoscale version of the operational Canadian Regional Finite-Element (RFE) Model with a grid size of 25 km is used to predict an intense MCS that occurred during 10?11 June 1985. The mesoscale version of the RFE model contains the Fritsch?Chappell scheme for the treatment of subgrid-scale convective processes and an explicit scheme for the treatment of grid-scale cloud water (ice) and rainwater (snow). With higher resolution and improved condensation physics, the RFE model reproduces many detailed structures of the MCS, as compared with all available observations. In particular, the model predicts well the timing and location of the leading convective line followed by stratiform precipitation; the distribution of surface temperature and pressure perturbations (e.g., cold outflow boundaries, mesolows, mesohighs, and wake lows); and the circulation of front-to-rear flows at both upper and lower levels separated by a rear-to-front flow at midlevels. Several sensitivity experiments are performed to examine the effects of varying initial conditions and model physics on the prediction of the squall system. It is found that both the moist convective adjustment and the Kuo schemes can reproduce the line structure of convective precipitation. However, these two schemes are unable to reproduce the internal flow structure of the squall system and the pertinent surface pressure and thermal perturbations. It is emphasized that as the grid resolution increases, reasonable treatments of both parameterized and grid-scale condensation processes are essential in obtaining realistic predictions of MCSs and associated quantitative precipitation.